1. Field of the Invention
The present invention relates to a communication system, as well as to a communication apparatus, which is adapted to perform large capacity data communication among information apparatuses, and particularly, relates to a communication system, as well as to a communication apparatus, which is adapted to perform data communication among information apparatuses by use of an electrostatic field without causing interference with other communication systems.
More specifically, an embodiment of the present invention relates to a communication system, as well as to a communication apparatus for transmitting radio frequency (herein after referred to as RF) communication signal between information apparatuses each of which is spaced apart in an ultra-close range, by use of an electrostatic field and/or an induction field, and more particularly, to a communication system, as well as to a communication apparatus, which allows large capacity transmission by use of an electrostatic field and/or an induction field with each apparatus arranged in an ultra-close range, by efficiently transmitting RF signals between an electric-field-coupling antenna (herein after referred to as EFC antenna) mounted to the respective information apparatuses.
2. Description of Related Arts
As methods of cable-less data transmission between small apparatuses, radio communication systems adapted to transmit/receive radio signals by use of an antenna have been developed, inclusive of Bluetooth communications and wireless LANs (Local Area Networks) representative of IEEE802.11. However, since radio communication by use of the antenna emits radio waves from a transmitting side, regardless of whether there exist the other party for making communication or not, a problem exists in that a source of radio disturbance against communication systems which are arranged in close range each other is caused. Further, the antenna also receives radio waves inclusive of those transmitted from a distant place, and hence, is easily affected by disturbing radio waves around, whereby reception sensitivity is degraded. In the presence of a plurality of other party for making communication, complicated setting is required to select desired one from a plurality of other parties. For the radio communication made by a plurality of pair of radio equipment within a limited range, frequency should be selected before the communication is made in order to avoid mutual interference. Further, a change of polarized wave direction to an orthogonal direction results in a failure of the radio waves communication.
On the other hand, communication systems using an induction field, an electrostatic field and the like may be employed in the radio communication, in addition to radio communication by use of a radiant electromagnetic field. The radio communication by use of the radiant electromagnetic field encounters a difficulty in realizing high-speed large-capacity feeble radio communication, because the electric field strength is inversely proportional to a distance and attenuation of the electric field strength due to the distance is relatively small. In contrast to the above, the induction field and the electrostatic field provide the electric field strength inversely proportional to the square and the cube of the distance respectively, thereby enabling the feeble radio communication such that the electric field strength (the intensity of radio waves) at a position three meters away from the radio equipment becomes a predetermined level or below, while maintaining the electric field strength for performing stabled radio communication with respect to the ultra-close range. Therefore, a radio station license is not required. Hence, non-contact ultra-close range communication by means of capacitive coupling using the induction field or the electrostatic field may be suited for realization of the feeble radio communication.
For instance, according to the communication system based on the capacitive coupling, other communication systems are not interfered, because no coupling relation is created and the radio wave is not emitted in the absence of the other party for making communication in a close range. Further, even if the radio wave is transmitted from a distant place, an EFC antenna makes no radio wave reception unless any capacitive coupling effect arises. Thus interference by the other communication systems may be prevented. More further, in the related art radio communication using the antenna, while the electric field strength of the radiant electromagnetic field is inversely proportional to the distance, in the induction electromagnetic field, the electric field strength is attenuated by the amounts inversely proportional to the square of the distance, and in the electrostatic field, the electric field strength is attenuated by the amounts inversely proportional to the cube of the distance. For this reason, according to the communication system based on the capacitive coupling, it is possible to provide the feeble radio communication assumed to be a level approximately equivalent to noise for other radio systems exists in a close range without requiring the radio station license.
Data communication technologies using the induction field and/or the electrostatic field have been widely used in radio frequency identifications (RFIDs) or the like. For instance, a proposal is made on a RFID tag system which enable secure reading/writing of information even though the RFID tags are overlapped. The above system is enabled by forming an auxiliary communication structure unit provided such that RFID tags are located between a plurality of auxiliary communication structure, and by arranging the RFID tags attached to a plurality of products such that they are sandwiched between the auxiliary communication structures (See Japanese Patent Application Publication No. JP2006-60283, Patent document 1, for instance).
There has been also a proposal on a data communication apparatus by use of an induction field, specifically, a data communication apparatus having, in addition to an apparatus body and attaching means to attach the apparatus body to a user's body, an antenna coil and data communication means adapted to perform data communication in a non-contact manner with an external communication apparatus with the antenna coil inbetween, in which the antenna coil and the data communication means are arranged in an outer case provided at an upper part of the apparatus body (See Japanese Patent Application Publication No. JP 2004-214879, Patent document 2, for instance).
There is further a proposal on a mobile telephone having a RFID which ensures a certain communication distance without degrading characteristics of mobile telephones by taking a structure in which an antenna coil adapted to perform data communication with an external apparatus is mounted to a memory card to be inserted into a mobile information apparatus and a RFID antenna coil is placed at the outside of a memory card insertion slot of a mobile information apparatus (See Japanese Patent Application Publication No. JP 2005-18671, Patent document 3, for instance).
The related art communication based on the capacitive coupling is unsuitable for transmission of a large capacity of data because low frequency signals are used. However, large capacity transmission may be attainable by transmitting RF signals by use of the capacitive coupling. Specifically, applications of high-frequency wide-band communication system, such as UWB communication to the capacitive coupling make it possible to realize large capacity data communication with the feeble radio communication.
The UWB (Ultra Wide Band) communication specified herein is adaptable to realize, by using extremely wide range of frequency bands, 3.1 to 10.6 GHz, large capacity radio data transmission, approximately 100 Mbps, for the close range (specifically, large capacity data, such as moving images and music data equivalent to data contained in a sheet of CD, for instance may be transferred quickly in a short time). Further, progress is extensively made in developing a transmission system by use of UWB low bands in the range of 3.1 to 4.9 GHz, in considerations of not only transmission practicability of data exceeding 100 Mbps without requiring the transmission band as wide as 3.1 to 10.6 GHz, but also easy fabrication of RF circuits.
The UWB communication, although originally being communication technology developed as the radio communication system using the antenna, in an embodiment of the present invention, it realizes high-speed data transmission by use of a feeble electric field with respect to a close range area. According to the UWB communication system by use of the electrostatic field, the high-speed data transmission by use of the feeble electric field with respect to the close range area may be realized, resulting in contributions to one of effective radio communication technologies loaded in mobile apparatuses, such as ultra high-speed close-range DANs (Device Area Networks) including storage devices, for instance.
When effecting not the radio communication but the UWB communication based on the capacitive coupling, the electric field strength is inversely proportional to the cube of the distance. For this reason, an attempt to control the electric field strength (the intensity of radio waves) at a position three meters away from the radio equipment at a predetermined level or below may provide the feeble radio communication requiring no radio station license to allow a communication system to be constructed at low cost. Further, for the data communication covering in the ultra-close range by use of the capacitive coupling, there are advantages in that signal quality is not degraded due to reflective objects around, and there is no necessity to give consideration to hacking prevention and/or privacy security on a transmission line.
FIG. 16 shows one configuration of a communication system composed of a transmitter and a receiver each of which is equipped with a RF EFC antenna. Referring to FIG. 16, a transmitter and a receiver are formed by connection of planar electrodes 14 and 24, serial inductors and parallel inductors to a RF signal transmission line. The RF signal transmission line specified herein includes coaxial cables, micro-strip transmission lines and coplanar waveguides etc.
The transmitting and receiving electrodes 14 and 24 respectively included in the transmitter 10 and the receiver 20 are placed face-to-face with around three centimeters distance kept, for instance, thereby allowing the capacitive coupling. When a host application sends a transmit request, a transmitter-side transmitting circuit unit 11 generates a RF transmitter signal, such as UWB signals based on transmission data, followed by propagation of the signal from the transmitting electrode 14 to the receiving electrode 24. Then, a receiver 20—side receiving circuit unit 21 performs modulating and decoding to the received RF signal, followed by transfer of reconfigured data to the host application.
As shown in FIG. 16, when EFC antennas included in the transmitter and the receiver are respectively faced each other, two electrodes act as a single capacitor and, on the whole, operate like a band-pass filter, thereby allowing the RF signals to be transmitted efficiently between the two EFC antennas.
Hereupon, in the communication by use of high frequencies covering GHz-bands, like the UWB communication, reflection of the signal causes a propagation loss when there is mismatch of impedance at a coupling part, leading to a reduction in efficiency. For preferable formation of a transmission line based on the capacitive coupling, sufficient impedance matching needs to be taken between the EFC antennas respectively included in the transmitter and the receiver.
Further, the communication system by use of the capacitive coupling also involves a problem of large attenuation depending on the distance. For the reason that the propagation loss is caused depending on propagation distance range in relation to a wavelength, the problem of the propagation loss depending on electrode-to-electrode spacing becomes serious during propagation of the RF signal by use of the capacitive coupling. Accordingly, the need exists to bring the coupling electrodes (the EFC antennas) of the transmitter and the receiver closer to each other as much as possible, causing a failure to communicate for far distance, resulting in a lack of user's convenience.
Subtle alignment of positions is required between the coupling electrodes to bring the coupling electrodes having the capacitive coupling sufficiently closer to each other, and during data communication, the positions need to be kept. There are many details difficult for the user to understand in that at which positions the electrodes should be provided in communication apparatuses, which electrode portion is required to be contacted, or how angle of faced electrode portions should be determined to obtain an optimum communication situation. Accordingly, there is a fear of failing to attain the maximum communication speed.